This invention relates to the detection of low frequency variations and high frequency cyclical variations in a signal, particularly those from on-line thickness measurement instruments.
Continuous processing of strip products such as steel, aluminum, rubber, paper, etc., has grown enormously over the past three decades. The quality of on-line instrumentation has improved correspondingly, and manufacturers are able to measure and control their processes within tighter and tighter tolerance bands. In many processes there are prolonged regions of high frequency variations in the product. An example is the thickness variation that occurs in the cold rolling of steel--in particular the phenomenon known as "chatter". A common cause of chatter is a mechanical resonance in the rolling mill which makes the rolls bounce. This gives rise to a thick (or thin) spot in the steel for each bounce. The thickness variations are a quality defect which may render the steel unsuitable for an end use such as can making.
Chatter may be characterized by the amplitude of the thickness variation and the spatial separation of the thick spots. For the roll bounce chatter described above, separations of 5-20 cm and amplitudes of 5-20 microns are found in 0.25 mm can stock. Such can stock is rolled at speeds from 600 to 2100 meters/minute, and finished on inspection, tinplating, and slitting lines at speeds from 300 to 600 meters/minute. The chatter shows up as high frequency variations in the range of 5 to 300 Hz. Regions of chatter may be as short as ten meters or as long as hundreds of meters. FIG. 1 shows an example of chatter, detected on a tinplating line with the strip of material moving at 500 meters/minute. The chatter is superposed on a much lower frequency variation in thickness, typically less than 2 Hertz.
Increasing concern on the part of the manufacturers has created the need to identify chatter on-line as part of the thickness measuring procedure. By identifying the chatter on a rolling mill it is possible to correct the cause of the chatter, and to identify the location of chatter within a coil of steel. On finishing lines the chatter can be cut out, or identified within the coil.
In conventional thickness gauges the thickness signals are integrated or filtered to improve the precision of the measurement. This smooths out the signal and destroys the chatter information. The present invention analyzes the signal prior to this integration and provides an output signal which identifies the presence of cyclical thickness variations such as chatter. The invention also provides means for identifying the amplitude and frequency of the chatter, and for calculating the spatial "wavelength" of the chatter. Such devices are not presently available, and the present invention represents a new type of instrumentation for on-line analysis of chatter.
While the example given above and the specific embodiments disclosed herein are for measurement of steel strip using a non-contacting nuclear thickness gauge, many other applications are possible using other non-contacting or contacting gauges, materials other than steel and properties other than thickness. The chatter detector is applicable to any continuous process in which there are high frequency signal variations. A very different example is measurement of optical density of liquids flowing through a pipeline.